The return of Mr Altman could potentially usher in a new era for the startup which had long juggled concerns among staff about AI's dangers and its potential for commercialisation. The original board had given scant explanation for Mr Altman's firing last week other than his lack of candour and its need to defend OpenAI's mission to develop AI that benefits humanity.

Analysts said the reshuffle will favour Mr Altman and Microsoft, which has pledged billions of dollars to the startup and is rolling out its technology to its customers globally. "There are still huge questions about why Mr Altman was fired and why Microsoft had been kept in the dark about the decision," said Danni Hewson, AJ Bell's head of financial analysis. Ms Hewson said: 

What does seem clear is that Microsoft will now play a much bigger role, that the partnership will become stronger and the two companies more integrated.

Microsoft's CEO Satya Nadella welcomed the changes. Microsoft shares rose 1.5%. 

It was not immediately clear if the previous board directors who hold no equity in OpenAI would retain their seats, or if the backers of its capped-profit subsidiary - such as 49% owner Microsoft - would ultimately win board appointments.

Unlike most Silicon Valley startups, OpenAI is overseen by a nonprofit parent board designed to ensure AI safety is given priority alongside growth. It created the capped-profit unit in 2019 to raise funds and grant stock options to its employees.

"The return of Altman consolidates his influence over the direction of OpenAI, and probably means it will be more bold and profit focused, but also potentially less risk averse," said Kyle Rodda, analyst at Capital.com.

Reuters earlier reported some shareholders were exploring legal recourse after the turmoil threatened the future of OpenAI, recently expected to have an over $80bn valuation.

Tuesday's moves reassured some investors. "We believe this is the best outcome for the company," said Thrive Capital, a backer of OpenAI. Mr Altman's dramatic turnaround drew comparisons in Silicon Valley lore to Steve Jobs, Apple's CEO who left the computer maker in a 1985 power struggle only to return 12 years later. Mr Altman took back the CEO mantle after four days. 

His departure triggered a major upheaval at OpenAI, with president Greg Brockman quitting in protest. 

By Sunday, Mr Altman was back at OpenAI's offices expecting his swift reappointment, when the board surprised again by naming ex-Twitch boss Emmett Shear as interim CEO.

Mr Altman's master stroke was made possible in part by Microsoft. When he was out of a job, chief executive Satya Nadella said Mr Altman could head a new research team alongside Mr Brockman and other colleagues departing from OpenAI. 

By Monday, nearly all of OpenAI's over 700-strong staff had threatened to leave and join Microsoft's effort unless the board stepped down and reinstated Mr Altman. 

This threat was backed by Microsoft's vast computing power, the key asset driving OpenAI's technology along with its staff of computer scientists. 

• Reuters